In order to prevent a temperature-control valve from being accidentally turned to a position in which it would deliver scalding water or to prevent a flow-control valve from being set in a position so far open that water is wasted, it is standard to provide an end stop that limits travel of the valve handle. Since there are however situations when scalding water and/or very heavy flow is desired, it is therefore also standard to provide an override mechanism so that the user can intentionally move past the normal end position.
As described in U.S. Pat. No. 4,767,052 and EP 0,232,454 such systems have a valve-actuating knob pivotal about and centered on an axis. A rocker is pivoted in turn on the knob about an axis that extends tangentially or secantally of the axis of the knob, so that the rocker itself extends parallel to the knob-rotation axis. An end of this rocker is depressed to move an abutment formed by the rocker on the knob out so it cannot engage an abutment formation on a sleeve or spindle carrying the knob.
Such a system is relatively effective but makes it difficult to design a compact knob and valve assembly. In addition accommodating the rocker requires that the entire knob assembly be relatively long, posing design constraints on the fixture.